The invention relates to a process by which biomass (cellulose, hemicelluloses, lignocelluloses, or any combination thereof) can be dissolved and converted into simple reducing sugars (glucose, fructose, glyceraldhyde, lactose, arabinose, and maltose) and other dehydration products using an appropriate ionic liquid solvent and an ionic liquid catalyst.
Cellulose is the most abundant biopolymer in the world and represents a tremendous quantity of stored energy. However, the properties of cellulose (high molecular weight, solid at room temperature, and solubility) greatly limit access to this stored energy. The energy potential of cellulose is essentially locked in the glucose units that make up the biopolymer, and thus the key to using cellulose as a renewable energy source is breaking the β-glycosidic bonds producing reducing sugars which can be used as inexpensive feedstocks in fermentations and hydrogenations leading to non-petroleum based fuels. The obstacle in using cellulose is the low yield of reducing sugars using traditional hydrolysis methods employing either enzymes or mineral acids. The low reactivity of cellulose is due to the hydrogen bonded supramolecular structure of the biopolymer. The structure restricts access to the glycosidic bonds resulting in poor yields. Several methods have been explored to overcome this problem.
The current aqueous based process for conversion of cellulose, purified biomass, to glucose involves a simple hydrolysis carried out in cellulose/water slurry with an enzyme or dilute mineral acid catalyst. These reactions result in the production of glucose in low yields. The reactivity of the cellulose is limited due to its highly crystalline nature. The crystalline regions of cellulose restrict the ability of the catalyst to access the glycosidic bond between the polymer units. The reactivity of the cellulose can be controlled by the ratio of crystalline regions to amorphous regions within the structure of the polymer. A higher crystalline ratio limits the solubility and reactivity of the polymer, since it is the amorphous regions that grant access to the glycosidic bonds. In this way the catalyst slowly degrades the polymer by digesting the amorphous regions first leaving behind an essentially crystalline matrix. One of the major drawbacks to this method is the incomplete digestion of the cellulose resulting in low yields of glucose. Another significant limitation of the use of mineral acids to convert cellulose to glucose is the further degradation of the glucose to products such as hydroxymethylfurfural, levulinic acid and formic acid.
Current methods use a pretreatment of the cellulose before processing to break up some of the crystalline regions increasing the percentage of amorphous regions in the cellulose; this, in turn, speeds up the depolymerization to glucose and improves the overall yield. There are several methods of pretreatment (biological, physical, chemical, and physiochemical) and each has its drawbacks. However these pretreatments are expensive, because they use costly solvents, are energy intensive, or are time consuming. Recently ionic liquid based pretreatment has shown potential as a cost effective alternative to aqueous based pretreatments in both enzymatic hydrolysis as well as acid hydrolysis.
Current use of Ionic Liquids in the Conversion of Cellulose to Reducing Sugars
One of the first applications of ionic liquids in this area was as a pretreatment of cellulose for enzymatic hydrolysis. In the case of A. P. Dadi, S. Varanasi and C. A. Schall, Biotechnol. Bioeng., 2006, 95, 904-910, the pretreatment resulted in amorphous cellulose that exhibited enhanced enzymatic reactivity. Recently ionic liquids have been used as a solvent for cellulose in traditional acid catalyzed hydrolysis. See, e.g., C. Li, Q. Wang and Z. K. Zhao, Green Chem., 2008, 10, 177-182. These studies demonstrated the benefits of using an ionic liquid in the hydrolysis of cellulose. The dissolution of the cellulose allows for increased reaction rates due to the accessibility of the glucosidic bonds in the cellulose. One of the drawbacks to this method is the use of concentrated mineral acid, in one case 98% wt. H2SO4, which requires care in handling and is volatile. Another report suggested the use of ionic liquids, again as a solvent for cellulose, but with solid acid catalysts for the hydrolysis of cellulose. While the solid catalysts demonstrated the ability to convert cellulose to simple sugars, heterogeneous catalysis can have low yields due to inefficient mixing.
This invention was developed to incorporate the acid site within the ionic liquid structure to provide a homogeneous catalyst. The ionic liquid catalyst structure can be modified to tune the catalytic activity and optimize the product mixture to obtain glucose with very little byproducts produced.